San Juans’ small mammals under the microscope | Guest Column
August 9, 2011 · Updated 5:00 PM
On nearly all of the San Juan Islands there are small mammals — voles, shrews, bats and mice are found on all of the islands.
However, Orcas and Lopez are the only island to have squirrels that are not flying squirrels. How did they get here? Are they the same as little critters on the mainland?
Not much is known about these island residents.
Ten thousand years ago our islands were being carved by glaciers. Since then, the small mammals must have colonized the islands from the mainland. It is not too hard to imagine how a bat may have been blown off course and ended up out on the islands, but how did the voles and shrews get here?
They cannot swim the distances needed. The best guess is that they floated across on vegetation. Chances are we will never know how they arrived here, but it may be possible to know when.
By looking at the genetics of our island’s populations and the mainland’s population it should be possible to see when their last common ancestor lived.
Isolated on islands, populations of animals tend to become unique, like the giant tortoises found in the Galapagos. Our small mammals may not be as impressive, but they may differ in appearance or genetics from their mainland relatives. San Juan and Shaw Island voles are described as a separate subspecies, for example.
It has not been confirmed, but would be interesting if true. Nothing in known about Orcas Island’s vole and shrew population.
Voles are found on nearly all of the San Juan Islands. They spend most of their time chewing, digging, and tunneling around native plants. This helps move seeds and root fragments, making new gardens for the voles. They behave basically like furry little land scrapers. Within the islands they are a major distributor of native plants. They are also seen as a nuisance by many human gardeners.
On the mainland, many different mammals and birds serve the function of distributing seeds and bulbs, helping plants spread. Here in the islands, dispersers are in short supply.
Northern Flying Squirrels live only in the woodlands on the west side of San Juan Island. Chipmunks live only on Lopez, also recently introduced gray squirrels. Stellar’s Jays are mainly found on Orcas.
Voles are the most widespread and populous of the islands’ dispersers. It is possible that they differ a little from island to island, however, even though they look very much alike.
Shrews and bats are our other widespread small mammals. Since they are shy and nocturnal, we see less of them and understand them even less than our voles and squirrels. But they also serve an important function: controlling insects.
Bats catch insects on the fly, literally, and eat not only mosquitoes and midges but moths as well. Shrews eat small ground-living animals such as beetles, grubs, and snails. Seven bat species have been reported so far, but we have only been able to find a single species of shrew.
The best and most humane way to study small animals is with those that are already dead. So, if you have a feline hunter, or find a deceased small mammal, please contact me at 370-5015 or firstname.lastname@example.org.
— Geneva Mottet is a university student and Beach Watcher working at Friday Harbor Labs, and studies small mammal biology at Lopez Island-based Kwiaht, a conservation laboratory.